Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high speed, high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having a diameter of at least 0.6 .mu.m and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional sensitizations, such as reduction sensitization were also disclosed.
An early, cross-referenced variation on the teachings of Kofron et al was provided by Maskasky U.S. Pat. No. 4,435,501, hereinafter referred to as Maskasky I. Maskasky I recognized that a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, adsorbed to the surfaces of host tabular grains was capable of directing silver salt epitaxy to selected sites, typically the edges and/or corners, of the host grains. Depending upon the composition and site of the silver salt epitaxy, significant increases in speed were observed. The most highly controlled site depositions (e.g., corner specific epitaxy siting) and the highest photographic speeds reported by Maskasky I were obtained by epitaxially depositing silver chloride onto silver iodobromide tabular grains. Maskasky I taught a preference for epitaxially depositing a silver salt having a higher solubility than the host tabular grains, stating that this reduces any tendency toward dissolution of the tabular grains while silver salt is being deposited. Maskasky I recognized that even when chloride is the sole halide run into a tabular grain emulsion during epitaxial deposition, a minor portion of the halide contained in the host tabular grains can migrate to the silver chloride epitaxy. However, the concentration in the epitaxy of any halide derived solely from the host tabular grain cannot be higher in the epitaxy than it is in the adjacent portion of the host tabular grain.
While Kofron et al and Maskasky I both acknowledged the possibility of very large average tabular grain sizes, ranging up to 10, 20 or even 30 micrometers (.mu.m), in fact, upon extensive further investigation, the art has adopted 10 .mu.m as an upper limit for the mean ECD's of tabular grain emulsions, and 5 .mu.m has emerged as a preferred maximum mean ECD, as illustrated by the teachings of Goda U.S. Pat. No. 4,775,617, Ikeda et al U.S. Pat. No. 4,806,461, Bando U.S. Pat. No. 4,839,268, Momoki U.S. Pat. No. 4,914,010, Saitou et al U.S. Pat. No. 4,977,074, Bell et al U.S. Pat. No. 5,132,203, Tsaur et al U.S. Pat. No. 5,210,013, Antoniades et al U.S. Pat. No. 5,250,403, Kim et al U.S. Pat. No. 5,272,048, Sutton et al 5,334,469, Black et al U.S. Pat. No. 5,334,495, Maskasky U.S. Pat. Nos. 5411,851 and 5,418,125, Delton U.S. Pat. No. 5,460,934, and Wen U.S. Pat. No. 5,470,698. With a high level of consistency patent Examples of tabular grain emulsions show mean ECD's well below 5 .mu.m.
The reason the art abandoned interest in tabular grain emulsions having mean ECD's above 10 .mu.m and has established 5 .mu.m as a preferred upper limit of tabular grain mean ECD's is that as mean ECD's are increased above 5 .mu.m higher granularities are observed, but no further increases in speed are observed. Thus, tabular grain emulsions are subject to the same losses in imaging efficiency with increasing grain sizes that have long been known to photographic scientists--that is, while speed and granularity increase in a predictable relationship up to a maximum speed, further increases in grain size merely increase granularity without increasing photographic speed. Reports of these observations are illustrated by Farnell, "The Relationship Between Speed and Grain Size", The Journal of Photographic Science, Vol. 17, 1969, pp. 116-125, and Tani, "Factors Influencing Photographic Sensitivity", Journal of Photographic Science and Technology, Japan, Vol. 43, No. 6, 1980, note particularly FIG. 1.
When working with emulsions differing in mean ECD below the mean ECD that produces maximum speed, the "predictable relationship" referred to above is this: If each stop (0.3 log E, where E is exposure in lux-seconds) increase in speed is accompanied by a granularity increase of 7 grain units, the emulsions in a series being compared are considered to exhibit equal photographic efficiency. For example, assigning a relative log speed of 100 to a reference emulsion, when an emulsion of a higher mean ECD exhibits a relative log speed of 130 (each unit difference in log speed=0.01 log E) and exhibits a granularity that is increased by 7 grain units, the two emulsions are exhibiting the relationship in performance that the art has established to exist between emulsions of the same photographic efficiency.
From the definition of COV above it is apparent that the standard deviation of grain size increases as the mean ECD of an emulsion increases. This in turn translates into lower levels of contrast, absent other adjustments. Thus, an additional disadvantage encountered in attempting to prepare progressively higher ECD tabular grain emulsions is that contrast declines as the mean ECD increases, even when COV remains constant. Thus, even if the speed barrier noted above could be overcome, the reduction in contrast would also restrict the range of photographic applications that can be served by higher tabular grain emulsions.